1. Field of the Invention
This invention relates to an apparatus for the growth of compound single crystals containing a component element having a higher vapor pressure (which will hereinafter be referred to as a "volatile element") by the Czochraliski method.
2. Description of the Prior Art
The hot wall method is known as a method for the Czochralski growth of single crystals using a molten B.sub.2 O.sub.3 seal ("Journal of Crystal Growth" 19 (1973) 356-358; "Journal of Crystal Growth" 24/25 (1974) 376/358; Japanese Patent Application OPI (Kokai) No. 123585/1979). In this method, a sealed vessel 1 having a structure as shown in FIG. 1 or FIG. 2 is filled with a vapor of volatile element such as arsenic and pulling is carried out in sealed vessel 1. Sealed vessel 1 is provided inside with crucible 3 containing a raw material melt 2 and pull rod 4 for pulling upward with rotation, to the lower end of which seed crystal 5 is fitted. Referring to FIG. 1, sealed vessel 1 is divided into an upper part and lower part at the side wall and provided with seals 6 and 7 at the opening part of the upper part and lower part and at the neck through which pull rod 4 passes. Referring to FIG. 2, sealed vessel is divided at the bottom and provided with seals 6' and 7 at the opening part of the divided parts and at the neck through which pull rod 4 passes. In both the cases, the seals consist of B.sub.2 O.sub.3 melt 8. Seed crystal 5 is immersed in raw material melt 2 and wetted therewith, and the crystal is grown and pulled up with revolving seed crystal 5.
These methods have a drawback in that because a quartz vessel is used as a sealed vessel 1, B.sub.2 o.sub.3 melt 8 reacts with the quartz and there is a large difference in coefficient of thermal expansion, resulting in breakage of the quartz vessel at the contact area with the B.sub.2 O.sub.3 melt. In addition, the raw material melt 2 is contaminated with silicon from the quartz vessel according to the following reaction: EQU 4Ga (in GaAs melt)+SiO.sub.2 (s).revreaction.2Ga.sub.2 O(g)+Si (in GaAs melt)
thus resulting in Si contamination of the pulled single crystal and deterioration of the electrical properties.
When there is a difference in pressure between the inside and the outside sealed vessel 1 and pressure balancing is lost, the liquid surface of the liquid seal 8 is fluctuated. In an extreme case, the liquid seal 8 itself falls from sealing portions 6 and 7, resulting in breakage and incompleteness of the sealing. That is, when seal 8, charged before pulling, falls from sealing portions 6 and 7 to break the sealing, volatile component elements such as arsenic leak out and pulling is thus impossible. Consequently, it is necessary to charge again the raw materials for the single crystal and seal.